Inflatable microwave antenna with variable parameters



May 23, 1967 K. IKRATH ETAL Filed Jan. 2'7, 1965 MWlN l 1 United States Patent Ofifice 3,321,763 Patented May 23, 1967 3,321,763 INFLATABLE MHIRSWAVE ANTENNA WITH VARIABLE PARAMETERS Kurt Ikrath, Elberon, and Wilhelm A. chneider, Fair Haven, N..I., assignors to the United States of America as represented by the Secretary of the Army Filed Jan. 27, 1965, Ser. No. 428,586 8 Claims. (Ci. 343--754) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

The present invention relates to an inflatable antenna for microwaves and more particularly to such an antenna in which certain parameters thereof may be controlled by controlling the pressure therein and by movement of the antenna feed relative to the rest of the antenna. Briefly stated, the invention comprises a flexible, inflatable bag which is mounted in the aperture of a microwave antenna. The bag is inflated by filling it with a mixture of small dielectric particles or pellets and gas. The pellet-filled bag refracts the microwave energy to concentrate it into a narrow directional beam. By varying the gas pressure within the bag and/ or moving the antenna feed relative to the bag, the antenna beamwidth can be varied. Parameter variation can also be accomplished by varying the size and/or composition of the pellets. When not in use the pellets may be removed and the bag and pellets compactly stowed for future use.

It is therefore an object of this invention to provide a novel and useful inflatable microwave antenna.

It is another object of this invention to provide a microwave lens antenna with controllable characteristics.

These and other objects and advantages will become apparent from the following detailed description and drawings, in which:

FIG. 1 is a cutaway side view of an illustrative embodiment of the present invention, taken along a plane passing through the longitudinal axis, 8-8, of the antenna, and

FIG. 2 is an enlarged view of a portion of FIG. 1 to show the structural details thereof.

In FIGS. 1 and 2, the antenna is of generally conical shape. The antenna feed consists of a helix 14 wound on a plastic cylinder 18 which in turn is centrally supported on metallic bracket 13. The bracket 13 is adapted to slide inside of fiberglass tube 12. The coaxial cable 17 has its center conductor connected to the left end of helix 14 and its outer conductor to bracket 13. The helix is fed in the axial mode, that is, the helix dimensions are related to the operating frequency in such a way that the radiation pattern is along the helix axis (line 8-8) toward the open or right hand end of the helix. A helical antenna of this type is described and claimed in copending patent application Ser. No. 236,460, now Patent No. 3,- 262,120, filed on November 8, 1962 by the present inventors and assigned to the US. Government. Inserted over the end of fiberglass tube 12 is a generally conical solid dielectric 11. The truncated conical-shaped bag 5 forms an extension of the solid dielectric 11. The bag 5 is adapted to be filled with dielectric particles or pellets 6. The solid dielectric and the pellet-filled bag together form a generally conical lens for refracting the microwave energy emitted by the helix 14. The solid portion 11 occupies approximately one third of the altitude (the dimension along line 88) of the conical-shaped lens but much less than one-third of the total volume thereof, therefore most of the refractive effect is due to the pellet-filled bag 5. The purpose of the solid dielectric 11 is to provide a substantial support for the inflatable bag 5 and also to accurately aline it with the antenna feed as well as to refract the energy emitted by the antenna feed.

As shown in FIG. 2, the right hand end of fiberglass tube 12 is inserted into cavity 19 at the apex of the solid dielectric 11, so that the axis of tube 12 is coincident with the longitudinal dimension or cone altitude 8-8. An annular-shaped cavity 15 is formed in dielectric 11 to receive the tube 12 and hold it accurately in alinement.

In operation, the dielectric pellets can be blown into bag 5 through tube 9. A sufficient number of pellets is inserted so that the mass of pellets fills the bag to form, with the solid dielectric 11, a substantially conical dielectric lens.

In accordance with one feature of the invention, the refractive properties of the lens can be varied and controlled by varying the air pressure within bag 5. This can be accomplished by forcing air into tube 9, from a pump, not shown, or bleeding air off through valve 10. The pressure changes correspondingly change the density of the pellets and consequently the dielectric constant and index of refraction thereof. As a result, the directional charactertistics of the antenna or lens will change with pressure changes within the bag. Also, the pressure changes can be correlated with frequency changes in the transmitted or received waves to maintain a constant beamwidth despite frequency changes. Normally, with a dielectric of constant properties, the beamwidth would be frequency dependent. Further, the size and/or composition of the pellets 6 can be chosen to obtain optimum performance at a given frequency. The refractive properties of the lens may be further controlled and varied by pressurizing the bag with other gases besides air, having different dielectric constants, for example carbon dioxide.

The slidable antenna feed in conjunction with the fiberglass tube 12 changes the directivity or beamwidth of the energy applied to the dielectric lens and also varies the distance of the energy source from the conical dielectric. The presence of sidelobes in the radiation pattern depends on thi distance and further, the optimum distance for minimum sidelobes is frequency sensitive, therefore the adjustment of the antenna feed position can be correlated with the applied frequency to minimize sidelobes as Well as provide a-beamwidth variation conjointly with that provided by the pressure control of the bag 5.

When not in use, the pellets may be removed from the bag and the bag collapsed and stowed for future use. Instead of providing the valve 16' for bleeding air, a porous bag may be used, in which case air would have to be continually pumped into tube 9 to maintain air pressure higher than atmospheric within the bag. In one antenna designed according to the principles disclosed herein, the overall length of the antenna is 9 feet, the bag 5 being approximately 5 feet in length (along line 8-8) with a base diameter of 3 feet, tapering to 1 foot diameter at the base of the solid dielectric 11. The bag is made of polyvinyl plastic and the dielectric pellets are of Dylite expanded styrene and are approximately inch in diameter. The solid dielectric 11 is also composed of expanded styrene. The operating frequency is 4 to 6 kilomegacycles.

In summary, the present invention provides a versatile and inexpensive antenna which has several modes of variation to accommodate various operating conditions. The feed position, bag air pressure, pellet size and material may all be varied to suit different conditions of operating frequency and the desired radiation pattern.

While the antenna has been illustrated with a helical type feed other types of feeds may be employed. Other modifications are possible without departing from the inventive concepts disclosed herein. Accordingly, the invention should be limited only by the scope of the appended claims.

What is claimed is:

1. A directional microwave antenna comprising, a helical antenna slidably mounted within a cylindrical dielectric tube, said dielectric tube inserted into a recess at the apex of a conical-shaped solid dielectric, an inflatable bag attached to the base of said solid dielectric and forming an extension of the conical shape of said solid dielectric, said bag filled with pellets of dielectric material, and means to pressurize the interior of said bag.

2. A directional microwave antenna with variable characteristics, comprising a dielectric lens of generally-conical-shape, said lens comprising a solid portion near the apex of said conical shape and the remainder of said lens comprising an inflatable bag adapted to be filled with dielectric pellets and pressurized, said inflatable bag comprising the larger portion of said lens, and an antenna feed supported by said solid portion and adapted to direct microwave energy along the altitude of said conical shape toward the base thereof.

3. The antenna of claim 2 in which said antenna feed is movable along the altitude of said conical shape.

4. A variable microwave antenna comprising, an inflatable bag in the shape of a truncated cone, said bag filled with pellets of material capable of retracting microwaves and capable of changing its index of refraction in response to pressure changes within said bag, said bag being supported by the base of a solid dielectric of generally conical shape, said solid dielectric supporting near its apex an antenna feed adapted to launch microwaves in the general direction of said inflatable bag.

5. A variable microwave antenna comprising, an inflatable bag in the shape of a truncated cone, said bag filled with dielectric pellets capable of refracting micro- Waves and also capable of changing index of refraction in response to pressure changes, means to pressurize said bag, said bag being supported by the base of a solid dielectric of generally conical shape, said solid dielectric and said inflatable bag forming together a generally conical shape, the altitude of said bag being about three times that of said solid dielectric, and a helical antenna mounted in a recess at the apex of said solid dielectric and adapted to be moved along the altitude of said solid dielectric.

6. A microwave antenna with variable parameters comprising, an inflatable bag filled with dielectric pellets capable of retracting microwaves and also capable of changing index of refraction in response to pressure changes, means to pressurize said bag, and a microwave feed arranged to direct microwave energy toward said bag.

7. The antenna of claim 6 wherein the distance between said bag and said microwave feed is adjustable.

8. A directional microwave antenna comprising, a helical microwave feed mounted within a fiberglass tube, said tube inserted into a recess at the apex of a conical-shaped solid dielectric, an inflatable bag attached to the base of said solid dielectric and forming an extension of the conical shape of said solid dielectric, said bag filled with pellets of dielectric material, and means to pressurize the interior of said bag.

References Cited by the Examiner UNITED STATES PATENTS ELI LIEBERMAN, Primary Examiner. 

1. A DIRECTIONAL MICROWAVE ANTENNA COMPRISING, A HELICAL ANTENNA SLIDABLY MOUNTED WITHIN A CYLINDRICAL DIELECTRIC TUBE, SAID DIELECTRIC TUBE INSERTED INTO A RECESS AT THE APEX OF A CONICAL-SHAPED SOLID DIELECTRIC, AN INFLATABLE BAG ATTACHED TO THE BASE OF SAID SOLID DIELECTRIC AND FORMING AN EXTENSION OF THE CONICAL SHAPE OF SAID SOLID DIELECTRIC, SAID BAG FILLED WITH PELLETS OF DIELECTRIC MATERIAL, AND MEANS TO PRESSURIZE THE INTERIOR OF SAID BAG. 